1. Field of the Invention
The present invention is in the field of gas analysis and more particularly relates to a compact sample cell for use in measuring very small concentrations of a gas based on the absorption of light or other radiation as it passes through the gas within the sample cell.
2. The Prior Art
Certain gases have absorption bands that absorb so weakly that absorption can only be detected after the radiation has traveled a relatively long distance, perhaps tens of meters, through the gas. In other situations the gases of interest might have adequate absorption strengths but they must be detected in very low concentrations, typically in parts per million (ppm) or less, so that long path lengths are required in this situation also.
Practical gas analyzers for commerical use seldom incorporate very long path-length sample cells for such purposes primarily because of size limitations. Where the use of long path-length sample cells cannot be avoided, it is common to use mirrors to fold an optical beam so that the latter can traverse the sample cell a number of times.
A multi-path absorption cell was described by J. U. White, Journal of Optical Society of America, Volume 32, page 285 (1942). The essential parts of the White cell consist of three spherical concave mirrors all having the same radius of curvature and positioned to form an optical cavity. Utilizing the principle outlined in White's article, at least two companies have marketed a ten-meter multi-path cell, and one of the companies has also marketed a forty-meter multi-path cell.
In U.S. Pat. No. 4,756,622 Wong disclosed another approach for providing long path-lengths for measuring the absorption by a gas. In Wong's invention, light is made to travel through a limited volume of gas a large number of times. The light is placed on a closed optical path on which it circulates through the gas sample. After a desired number of passes through the gas sample, the light is removed from the closed optical path. Introduction of the light to the closed optical path and removal therefrom is accomplished through the use of a polarizing beamsplitter and a Pockels cell located on the closed path. Light is put onto the closed path by the polarizing beamsplitter which imparts a specific polarization. During the first circuit the Pockels cell alters the polarization by 90 degrees thereby preventing the light from escaping back out through the polarizing beamsplitter. After the desired number of circuits, the Pockels cell again alters the polarization by 90 degrees thereby permitting the light to be redirected out of the closed path by the polarizing beamsplitter.
While the White cell and Wong's invention work well in certain specific applications, they both suffer from the disadvantages of being complex and expensive. They are therefore not practical for use in low-cost mass-oriented applications such as in toxic gas monitors (e.g., carbon monoxide, ozone, nitric oxide, etc.) where low ppm detection is often required. Furthermore, in both of the above devices, the gas whose concentration is to be measured has to be introduced into the sample cell via a cumbersome vacuum pump.
The success of Non-Dispersive Infrared (NDIR) techniques in gas analysis has led the present inventor to consider the application of this technology to the design and construction of new fire detectors. The technique involves detecting the rapid build-up of low concentrations of carbon dioxide and carbon monoxide gases in the ambient air caused by the fires.
The concentrations of these gases that need to be detected lie typically in the tens to hundreds of ppm. Because concentrations are so low, long path-length sample chambers are required.
The complex and expensive multi-path sample cells mentioned previously are clearly not suitable for this type of application. A simple and low-cost long path-length sample cell, which is currently not available commerically to the best of our knowledge, is needed instead.